This invention relates to gamma cameras, and more particularly to a method of and means for improving the resolution of such cameras.
As is well known, a conventional gamma camera has a scintillation crystal responsive to radiation stimuli for producing light events in the crystal at locations where the stimuli interact with the lattice structure of the crystal. An array of photomultipliers operatively associated with the crystal responds to light events for producing individual outputs which are processed by computation circuitry that computes the coordinates of each light event.
In preparation for displaying the intensity distribution of a radiation field imaged on the crystal and producing light events therein, it is conventional to accumulate a representation of the density distribution of events in the crystal by utilizing a matrix of storage registers whose elements are in one-to-one correspondence with elemental areas of the crystal. Each time a light event occurs in the crystal, its coordinates are calculated, and the contents of the element in the matrix comprehending such coordinates is incremented. Thus, contents of a given element of the matrix is a number that represents the number of events that have occurred within a predetermined period of time within an elemental area of the crystal which corresponds to the location of the given element in the matrix. Such number is directly proportional to the intensity of radiation emitted from an elemental area of the radiation field with which the given element of the matrix is associated. Therefore, by using this number to establish the brightness of a picture element of a display corresponding to the elements of the matrix, the intensity distribution of a radiation field can be displayed in terms of the brightness distribution of the display.
In a conventional gamma camera of the type disclosed in Reference [1], the coordinates of a light event are computed by operating on the output signals of the photomultipliers. Specifically, the so-called "center of gravity" of the photomultiplier signals is computed by assigning a weight to each photomultiplier in accordance with its location relative to a coordinate axis in the crystal, multiplying the weight of a photomultiplier by its output, summing all of the weighted outputs of the photomultipliers, and dividing the sum by the number of photomultipliers. For reasons well known to those skilled in the art, a computed location of a light event may be displaced from its actual location in the crystal. As a consequence, an image produced from a matrix whose contents are developed in this manner is not always an accurate reproduction of the actual radiation field. Accuracy can be improved if compensation is made for non-linearities in the system.
One approach is disclosed in Reference [2] where the computation is done in a step-by-step process that involves first coarsely determining the general location of an event and then utilizing a predetermined function of the outputs of photomultipliers close to the general location in order to compute the coordinates of the event. Another approach is disclosed in Reference [3] wherein a calibration map or matrix is obtained for a given gamma camera and the computed coordinates are modified in accordance with the calibration map. In no case, however, have these expedients been fully successful in correcting the inherent problems in gamma cameras of accurately displaying an image of a radiation field.
It is therefore an object of the present invention to provide a new and improved method of and means for improving the resolution of a gamma camera of the type described above.